Bile acids are mainly produced in the liver with cholesterol 7a hydroxylase (cyp7a1) the rate-limiting enzyme. Bile acids are involved in liver, biliary, intestinal, and cardiovascular diseases. FXR is a bile-acid activated nuclear receptor and is essential in maintaining bile-acid homeostasis. FXR dysfunction contributes to the development of cholestasis, gallstones, fatty-liver disease, and liver tumor. Emerging evidence suggest that in mice FXR suppresses cyp7a1 gene expression via pathways initiated both in the liver and the intestine. However, to understand the roles of FXR in suppressing bile-acid synthesis, clear gaps exist in consolidating where the suppressive pathway initiates, to what extent SHP and FGF15 are involved, and what signaling molecules are in the liver to suppress cyp7a1 gene expression. The objective of this application is to establish the underlying molecular mechanism by which FXR regulates bile-acid synthesis in a tissue-specific manner. My hypothesis is that intestine-initiated FGF15- FGR4 pathway is the major and the liver-initiated SHP-LRH-1 cascade is a minor underlying mechanism for suppressing cyp7a1 with FXR activation, in addition, both Egr-1 and cJun are involved in FGFR4 pathway to suppress cyp7a1 gene expression. The hypothesis is based on our compelling preliminary data that collectively showed that FXR, but not SHP, intestinal FXR, but not hepatic FXR, is mainly responsible for the suppression of cyp7a1 gene expression. We plan to test the hypothesis and accomplish the objective by pursuing the following specific aims. (1) Establish the tissue-specific function of FXR in suppressing cyp7a1 gene expression. (2) Determine to what extent that SHP and FGF15 are involved in suppressing cyp7a1 and regulating bile-acid homeostasis. (3) Elucidate the signaling pathways for suppressing cyp7a1 gene expression following FGFR4 activation. The proposed research is innovative and represents a paradigm shift in understanding the regulation of bile-acid synthesis. We are the first to combine the usage of tissue-specific FXR KO mice with cellular and molecular techniques to identify the fundamental pathways in regulating bile-acid feedback inhibition. These studies will identify the tissue specific roles of FXR, which will provide a fundamental understanding for the study of bile-acid, cholesterol, and triglyceride homeostasis. Furthermore, completion of the proposed study will shed light on designing tissue-specific FXR modulators in the future to better prevent and treat human diseases associated with bile-acid disorders. Public Health Relevance: Bile acids are the main components in bile and are important for regulating cholesterol and triglyceride homeostasis. FXR is essential in regulating bile-acid homeostasis, and contributes to the development of cholestasis, gallstones, fatty-liver disease, liver tumors, and atherosclerosis. Completion of the proposed study will shed light on designing tissue-specific FXR modulators in the future to better prevent and treat human diseases associated with bile-acid disorders.